update cohere endpoints & response template

[b4s36t4]

Title: Deprecate /generate endpoint for cohere provider

• Updates cohere endpoints in favour of depreciating generate endpoint.

Motivation: (optional)

• Why this change is necessary or beneficial

Related Issues: (optional)
/closes #321

[narengogi]

I've checked out your branch and ran a test, but it fails.
Please find attached relevant curl request

curl --location 'http://localhost:8787/v1/chat/completions' \
--header 'x-portkey-provider: cohere' \
--header 'Authorization: $COHERE_API_KEY' \
--header 'Content-Type: application/json' \
--data '{
    "model": "command-r-plus",
    "max_tokens": 200,
    "stream": false,
    "messages": [
      {
        "role": "system",
        "content": "You are a helpful assistant"
    },
          {
        "role": "user",
        "content": "Hello Alfred, how are you?"
    }
    ]
}'